1. Field of the Invention
The present invention relates to a base station, a control device, a communication system, and a communication method.
2. Description of the Related Art
Requirements for a maximum information transmission rate in a moving environment, walking environment, and quasi-stationary environment of the next generation mobile communication system, i.e., International Mobile Telecommunication—2000 (IMT-2000) are 144 kbps, 384 kbps, and 2 Mbps, respectively. As a result, it becomes possible to implement real multimedia mobile communication in addition to voice service. Considering rapid spread of Internet, increases in the dimension of information, increases in the capacity of information, and development of the next generation Internet in recent years, however, there is a pressing need in the mobile communication system to develop a radio access scheme capable of implementing a transmission rate exceeding 2 Mbps. It is considered to be increasing especially in a radio link for down direction traffic required to be at a high rate and have a large capacity, such as for downloading an image, a file, or a dynamic image such as video data from a database or a Web site. Therefore, a high speed packet transmission technique suitable for traffic of high speed and large capacity is indispensable.
From the background as described above, a scheme for implementing high speed packet transmission with a maximum transmission rate of 2,4 Mbps based on a radio interface of IS-95 has been proposed (“CDMA/HDR: A Bandwidth—Efficient High-Speed Wireless Data Services for Nomadic Users,” P. Bender, P. Black, M. Grob, R. Padovani, N. Shindhushyana, and A. Viterbi, IEEE Communication Magazine, Vol. 38, no. 7, pp. 70-77, July 2000, which is hereafter referred to as first paper). Furthermore, in 3GPP (3rd Generation Partnership Project) as well, implementation of high speed packet transmission having a maximum transmission rate of approximately 2.4 Mbps obtained by expanding the W-CDMA (Wideband Code Division Multiple Access) radio interface is studied.
As for such high speed packet transmission, application of a technique, such as adaptive modulation and demodulation and error correction, based on adaptive radio link control (link adaptation), such as channel coding, proposed in “Symbol Rate and Modulation Level—Controlled Adaptive Modulation/TDMA/TDD system for High-Bit-Rate Wireless Data Transmission,” T. Ue, S. Sampei, and N. Morinaga, IEEE Transaction. VT, pp. 1134 - 1147, Vol. 47, no. 4, November 1988 is studied. In the adaptive modulation and demodulation and error correction, based on adaptive radio link control, the multivalue number of data modulation, spreading factor, the multicode multiplex number, and the coding factor of error correction are switched over according to the propagation environment of a user, in order to conduct high speed data transmission efficiently.
For example, as for data modulation, the QPSK (Quadrature Phase Shift Keying) modulation used in the current W-CDMA is switched over to a multi-value modulation having a higher efficiency, such as 8 PSK modulation, 16 QAM (Quadrature Amplitude Modulation) modulation, or 64 QAM modulation, as the propagation environment becomes favorable. As a result, the transmission speed of the communication system can be increased. For example, if data modulation using 64 QAM is conducted in the case where the spreading factor=4, the number of multi-codes=3, and the error correction coding factor=½, ultra-high speed data transmission of 8.5 Mbps becomes possible with a W-CDMA interface having a chip rate of 3.84 Mcps. As for the high speed packet transmission, application of the ARQ (Automatic Repeat request) technique proposed in “Automatic-Repeat-Request Error Control Schemes,” S. Lin, D. Costello. Jr., and M. Miller, IEEE Communication Magazine, Vol. 12, no. 12, pp. 5-17, December 1984 is also studied.
On the other hand, in order to conduct such packet transmission efficiently, there is proposed a communication method in which a base station that transmit and receive packets monitors the situation of data transmission paths formed between the base station and mobile stations that conduct communication with the base station, determines a mobile station to which packets should be transmitted, and assigns a channel. For example, in the above-described first paper, there is proposed a method of maximizing throughput in a sector or a cell by giving priority to transmission of a packet to a mobile station that is favorable in reception state and transmitting a packet preferentially to the mobile station.
However, this communication method causes unfairness that only a specific mobile station can conduct communication and other mobile stations cannot conduct communication. In order to reduce the unfairness, therefore, there is proposed a communication method of deriving a ratio of an average value of a transmission rate determined by a past transmission path situation to a current instantaneous transmission rate, and transmitting a packet to a mobile station that is large in the ratio (“Data throughput of CDMA—HDR a High Efficiency—High Data Rate Personal Communication Wireless System,” J. Jalali, R. Padovani, R. Pankaj, Proc. of IEEE VTC 2000—Spring, pp. 1854 - 1858, Tokyo, May 2000). This communication method has an advantage that unfairness among mobile stations is mitigated. In Japanese Patent Application Laid-Open Publication No. 2000-224231, there is proposed a packet data communication method of deriving priority every packet data and assigning channels earlier to packet data having high priority.
Once a base station has decided to transmit a certain packet based on the reception state and transmission rate, however, thereafter the base station assigns a channel to the packet and transmits the packet, in the conventional communication method. Also when deriving priority of transmission, once the base station has derived priority, thereafter the base station assigns a channel to the packet based on the priority. When actually transmitting the packet, however, there might occur a situation where a packet having low priority should be transmitted rather than a packet decided once to be transmitted or a packet having priority once derived. Even for a packet once decided to be transmitted or a packet having high priority derived once, there might occurs a situation where it is not necessary to transmit the packet immediately.
Even if such a situation occurs, the base station conducts channel assignment according to the priority once derived, in the conventional communication method. In the conventional communication method, therefore, the base station cannot conduct channel assignment according to the situation at the time of transmission without being restricted by the priority once derived. For example, if the reception state of a mobile station that is destination of a packet is not good when a base station transmits the packet, then the possibility that the transmission will fail is high even if the packet has high priority. This results in high possibility that radio resources will be wasted. From the viewpoint of preventing the radio resources from being wasted, therefore, it is preferable to transmit another packet rather than the packet having high priority. If priority derived once is high, however, then in the conventional communication method a channel is assigned to the packet though the reception state is not good, resulting in waste of radio resources.
In the case where there is a margin in permissible delay time needed in service quality of a packet when a base station transmits the packet, it is not necessary to transmit the packet immediately even if the packet has high priority. If there is another packet having high urgency, therefore, it is preferable to transmit the packet having high urgency preferentially. If priority derived once is high, however, then in the conventional communication method a channel is assigned to the packet though there is a margin in the permissible delay time and the packet is transmitted with a superfluous quality as compared with the permissible delay time.